a situation that occurs when two distinct pieces of data have the same hash value, checksum, fingerprint, or cryptographic digest.
- Stackoverflow.com Wiki
A hash function takes an item of a given type and generates an integer hash value within a given range.
Let's consider this: say you have a git repository, make a commit, and get very very unlucky: one of the blobs end up having the same SHA-1 as another that is already in your repository. Question is, how would git handle this? Simply fail? Find a way to link the two blobs and check which one is needed according to the context?
A few weeks ago, researchers announced SHAttered, the first collision of the SHA-1 hash function. Starting today, all SHA-1 computations on GitHub.com will detect and reject any Git content that shows evidence of being part of a collision attack. This ensures that GitHub cannot be used as a platform for performing collision attacks against our users.